The present invention relates generally to laser processing of materials. More particularly, the present invention relates to methods and apparatus employing a series of laser pulses which have been specifically shaped to provide better processing quality and higher throughput in laser processing applications. The present invention also relates to scribing of thin film materials on a substrate. However, the invention has broader applicability and can be applied to other applications and materials.
Pulsed laser sources, such as Nd:YAG lasers, have been used to perform laser-based material processing for applications such as marking, engraving, micro-machining, cutting, and scribing. One such process where lasers are commonly used is scribing lines in a thin film of material on a thicker substrate. A thin film is defined in very general terms as a layer of material which is only a few molecules thick. In practice, a thin film is typically between 25 nm and 2 microns in thickness. A substrate is a material upon which the thin film is deposited and typically the substrate is substantially thicker than the thin film. There are many examples of the use of thin films in areas such as electronic devices, electro-optical devices, optical devices, and corrosion protection. For example, photovoltaic or solar cells can have thin films of amorphous silicon, cadmium telluride, copper indium diselenide, copper indium gallium diselenide, or molydenum, and electrodes made using thin films of transparent conductive oxide (TCO) material such as indium tin oxide (ITO), zinc oxide (ZnO) and oxides of other metals such as aluminum or molybdenum. Thin films of these and other materials are also used in flat panel displays and digital displays.
Scribing a line in a thin film material on a thicker substrate means to remove all the thin film material down to the substrate and do this along a line. For relatively thick lines, a knife can be used but it often results in rough edges and incomplete removal of the thin film material. The width of the line required in electronic devices can be very thin. Lasers are used for the application of scribing lines in thin film materials because they can be used to cut a very thin line and cleanly ablate the thin film material.
When scribing TCO, one parameter that is monitored is the resistivity achieved across the scribed line. The resistivity is affected by the amount of TCO material which is removed in the scribing process and the goal therefore is to remove all the TCO material in the groove being cut. One issue can be the amount of residue and debris which is generated in the cutting process. Ablated TCO material can fall across the groove as it is being scribed and thereby reduce the resistivity. Even if this does not happen immediately, the presence of debris can result in a reduction in resistivity sometime later if the debris is swept into the groove. A goal of the manufacturing process is to minimize the amount of residue and debris. For this reason, laser scribing often takes place with the beam passing through the glass substrate so that it is a “second-surface” process; although this helps to reduce the amount of residue and debris which sticks to the surface, some residue and debris remains. A typical acceptable value of resistivity is 200 MegaOhm although the ideal value depends on the application.
Another issue which does affect the quality of the laser scribing process is the generation of micro-cracks in the glass substrate or in the walls of the TCO material in the scribed groove. Over time, micro-cracks can propagate and become bigger with the result that mechanical flaws can appear at or across the scribed groove. Such occurrences are to be avoided since they can lead to device failure at some time after the standard “infant mortality” test phase and are thus difficult to counteract. Any physical deterioration of the thin film or the substrate by the laser pulse must be minimized. If present, micro-cracks and residue and debris can be observed using a high power optical microscope.
Depending on the application and the materials to be processed, it can be advantageous to be able to select the various characteristics of the laser pulses, including pulse energy, pulse width, pulse repetition rate, peak power or energy, and pulse shape, as appropriate to the particular application. Many examples exist of the careful control of pulse energy and power to optimize various materials processing applications.
Many existing high power pulsed lasers that are characterized by pulse energies greater than 0.5 mJ per pulse, rely on techniques such as Q-switching and mode locking to generate optical pulses. However, such lasers produce optical pulses with characteristics that are predetermined by the cavity geometry, the mirror reflectivities, and the like. Using such lasers, it is generally difficult to achieve an optimal pulse shape for the application at hand and therefore in many cases the laser processing has some deficiencies.
Therefore, what is needed is a system and method for scribing thin films of materials that improve the quality and the yield of the thin film scribing process.